Electroless plating includes chemically reducing metal ions in an electroless plating solution onto a conductive or non-conductive surface without supplying any electric current from the outside. Electroless plating is widely used in nickel-phosphorus deposition, nickel-boron deposition, and a copper deposition. In particular, electroless copper plating can be used to form a copper film onto substrates in the fabrication of printing circuit boards and other electronic devices. Electroless copper plating is widely used because the deposition process is simple and the copper film is highly conductive.
Electroless plating can be accomplished either by immersion electroless systems or by spray electroless systems. In immersion electroless plating systems, the surface to be coated is immersed in the electrolyte bath. The reduction reaction is catalyzed by the seed layer, thereby increasing the metal thickness. By comparison, the electrolyte solution is sprayed over the object in spray electroless plating systems.
Electroless plating involves the formation of a thin film of material (i.e., a metal such as copper) from an electroless plating solution without external electric current. The electroless plating solution usually contains metal ions, a metal ion complexing agent, a reducing agent for reducing the metal ion to deposit the metal, and a pH buffer. In addition, the electroless plating solution may contain a stabilizer for improving the stability of the electroless plating solution, and a surfactant for improving the properties of the metal film.
Electroless plating occurs by two simultaneous half reactions involving electron generation and electron reduction. The metal cations in the solution accept electrons at the deposition surface, become reduced, and are deposited as metal on the surface of the substrate.
A catalytic surface usually consists of either a surface which has been activated, for instance with palladium-tin colloid, or a thin evaporated or sputtered seed of a noble metal like gold, platinum or palladium. Once a thin layer of metal has been deposited onto the seed layer or sensitized surface, electroless plating continues autocatalytically, since the metallic film is also a good catalyst for electroless growth.
However, electroless copper plating solutions typically use formaldehyde or its derivatives as reducing agents, which are volatile carcinogenic liquids. In addition, using formaldehyde requires that the electroless solution be operated at pH conditions of 11 or more. Thus, materials that are sensitive to higher (more basic) pH solutions cannot be used in electroless copper plating systems that include these types of chemicals in the electroless plating solutions.
Thus, a heretofore unaddressed need exists in the industry for a electroless solution that addresses the aforementioned deficiencies and/or inadequacies.